Rburization refractory iron base alloy resistant to high temperatures and to reca

ABSTRACT

AN IRON-BASE ALLOY HAVING A GOOD CREEP RESISTANCE AT HIGH TEMPERATURE, A GOOD CORROSION RESISTANCE UP TO AT LEAST 1,200*C. AND A HIGH RESISTANCE TO CARBON RESTORATION OR RECARBURIZATION BY CARBURIZING AGENTS AND CONTAINING CARBON, NICKEL, CHROMIUM, MANGANESE, SILICON, NIOBIUM WITH THE USUAL IMPURITIES AND WHEREIN THE IMPROVEMENT CONSISTS IN THAT SAID ALLOY CONTAINS 0.3 TO 4.5% OF TUNGSTEN, 1.0 TO 8% OF NIOBIUM AND 0.02 TO 0.25% OF NITROGEN.

United States Patent US. Cl. 75122 1 Claim ABSTRACT OF THE DISCLOSURE An iron-base alloy having a good creep resistance at high temperature, a good corrosion resistance up to at least 1,200 C. and a high resistance to carbon restoration or recarburization by carburizing agents and containing carbon, nickel, chromium, manganese, silicon, niobium with the usual impurities and wherein the improvement consists in that said alloy contains 0.3 to 4.5% of tungsten, 1.0 to 8% of niobium and 0.02 to 0.25% of nitrogen.

The present invention relates to an iron-base alloy as well as to work pieces and component parts made from such an alloy.

There is presently a need for industrial alloys likely to offer a high resistance to recarburization or carbon restoration by carburizing agents; this is for example the case for alloys used for making component parts for solid and gas carburizing or case-hardening furnaces or for petrochemistry ovens or furnaces in particular for cracking hydrocarbons through pyrolysis and other reforming processes.

Such alloys should offer a good creep resistance at high temperatures as well as a good corrosion resistance in particular to oxidation at a high temperature.

Finally these alloys should have a good thermal shock resistance and preferably a good weldability.

'Ihe presently known alloys which are most likely or adapted to resist creep and hot corrosion do not exhibit all of the above-mentioned characterizing features and among these the most important of all, namely the resist ance to recarburization by the carburizing agents.

Contrary thereto the alloy according to the present invention obviates the drawbacks of the well known alloys and may be used as a component material for furnaces or ovens since it exhibits all the above-mentioned properties at the same time.

The alloy according to the present invention is of the known kind containing carbon, chromium, nickel, manganese, silicon, niobium with the usual impurities and it is characterized in that it contains 0.3 to 4.5% of tungsten, 1.0 to 8% of niobium and 0.02 to 0.25% of nitrogen (weight ratios).

In the temperature range extending up to at least 1,200 C., this alloy exhibits high mechanical strength and low creep rate characteristics associated with a very good resistance to recarburization and to corrosion in general, in particular by oxidation.

This alloy further exhibits a good weldability and a good thermal shock resistance.

The aforesaid properties enable the use of said steels in furnaces where recarburization promoting conditions are combined, which recarburization does not take place, contrary to what occurred under the same circumstances, with previously known steels, especially when the temperature became higher than about 700 C.

The applicants could show that the outstanding resistance to carburization was due to the simultaneous presence of the tungsten, niobium and nitrogen elements in the above-mentioned proportions or relative amounts.

According to a preferred embodiment of the present invention, said alloy contains 0.5 to 3% of tungsten and 1.0 to 3% of niobium; abiding by these particular limits of composition, as regards tungsten and niobium, enables to obtain an alloy having a specially high weldability.

According to a further characterizing feature of the present invention corresponding to said preferred embodiment, the tungsten contents by weight ranges from 1.40 to 2.0% and that of niobium ranges from 1.0 to 1.8% by weight, thereby yielding the most etiicient steel from the standpoint of resistance to carburization, creep resistance, resistance to oxidation and welding easiness.

The weight ratios of elements other than tungsten, niobium and nitrogen may vary within fairly wide proportions. Preferably these proportions should be such that the alloy according to the present invention have the following composition, the percentages being expressed by Weight:

Percent Carbon 0.05 to 0.85 Chromium 20 to 35 Nickel 22 to 40 Manganese 0.2 to 4 Silicon 0.2 to 3 Tungsten 0.3 to 4.5 Niobium 1.0 to 8 Nitrogen 0.02 to 0.25 Phosphorus Below 0.05 Sulphur Less than 0.05 The remainder being iron with the usual impurities which should be present in the least or smallest possible residual amount.

In the case of a steel having the optimum welding characteristics, the weight ratios of the various components of the alloy according to the present invention are the following:

the remainder being iron with the usual impurities least or smallest possible residual amount.

Eventually the following composition is that corresponding to the optimisation of all the properties of the steel from the standpoint of resistance to carburization. creep resistance, resistance to oxidation and welding easiness: I

Percent Carbon About 0.40 Chromium About 25 to 28 Nickel About 32 to 36 Manganese About 0.50 to 1.0 Silicon About 1.20 to 1.60 Tungsten About 1.40 to 2.0 Niobium About 1.0 to 1:8 Nitrogen About 0.15 Phosphorus and sulphur 1 In the least possible amounts.

the remainder being iron with the usual impurities reduced to a minimum.

Further characterizing features of the invention will appear in the course of the following description of three examples the first of which relates to a creep test and the two others relate to carburization tests. In all these examples, the reference numeral I designates a steel according to the present invention, the other steels being not a part thereof.

EXAMPLE 1 Creep tests have been carried out at a temperature of 950 C. during 1,000 and 10,000 hours in order to appraise the action of alloy elements such as niobium, tungsten, nitrogen and molybdenum upon the properties under hot conditions of a basic alloy containing about 0.40% of carbon, 33% of nickel and 25% of chromium, the other elements being comparable.

The results obtained are specified in the following table:

TAB LE 1 Creep resistance in heetobars The results obtained show that the alloy I according to the invention exhibits one of the best creep resistances among the tested steels.

Other creep resistances by breaking under tension at higher temperatures in the range of 1,050 C. to 1,200 C. for very long times have also borne out the superiority of the alloy forming the subject matter of the present incarburization depth limit. These tests are summarized in the following Table 3 (the'steel according to the invention is the same as that of Example 1) TABLE 3 Carbon content after case-hardening at 1,150 C. to

Further carburization tests have been carried out in an industrial furnace which has been kept in use for a year at a temperature of l,050 C. and which contained composite tubes made of various alloys arranged the ones after the others so that they be subjected to the same temperatures and to the same carburizing gas. After segmenting of the tube or pipe, the carbon content has been determined for each alloy in successive layers with thicknesses of 0.5 mm. as well on tube portions remaining raw as on portions machined along the inner diameter. All the alloys subjected to the test were iron-base alloys containing about 0.40% of carbon, the alloy I according to the invention being the same as in Examples 1 and 2.

The following Table 4 specifies the percentages by weight or weight ratios of the other components of the alloys subjected to said tests as well as the increase in the carbon content of the second layer having a thickness of 0.5 mm. when starting from the inside of the tube as well as that corresponding to the middle of the thickness of the tube and this for each tested grade or quality:

TABLE 4 Increase in percent by weight in carbon content Raw tube Machined tube Composition in percent Inside Centre Centre at of thickof thick- Ni Cr Nb W Mo N 0.5 mm. ness Inside ness 1 Alloy I.

vention; the results of these further tests are given in Table 2 hereinafter (the basic alloy comprised also about 0.40% of carbon).

TABLE 2 Breaking time in hours at- Composition in percent by weight 1,050 0., 1,200 0., load 2 load 0.7 Nb W Mo N heetobars hectobar EXAMPLE 2 Carburization tests have been carried out with a batch of iron-base alloys (carbon content: about 0.40% by weight) by putting the samples of these alloys into cementation or hardening cases at elevated temperatures. After given periods of time, the carbon content of the case-hardened layers versus or against the depth was analyzed. The results have shown that the layers at a depth of about 0.5 mm. have become richer in carbon and that the layers at a depth of about 5 mm. set the The above table shows the significant improvement brought about by the new alloy as regards the raw tubes or pipes as well as the machined tubes, thereby confirming the results shown in Table 3 while moreover substantiating the detrimental influence of molybdenum.

The easy resistance to carburization of the alloys according to the present invention is due to the simultaneous presence of tungsten, niobium and nitrogen in the amounts stated hereinabove and this could be explained in the following manner:

When forming nitrides in particular with chromium, the nitrogen decreases the formation of chromium carbides and accordingly restrains the carbon penetration.

Tungsten and niobium form stable carbides which also impede the carbon penetration.

The present invention is of course not at all limited to the forms of embodiments described which have been given by way of example only. In particular it comprises all the means forming technical equivalents to the means described as well as their combinations if same are caried out. according to the gist of the invention.

by carburizing agents, said alloy having the following 5 composition:

Percent by weight Carbon About 0.40 Chromium About 25 to 28 Nickel About 32 to 36 Manganese About 0.50 to 1.0 Silicon About 1.20 to 1.60

Tungsten About 1.40 to 2.0 Niobium About 1.0 to 1.8

Nitrogen About 0.15

Percentage by weight Phosphorus Below 0.05 Sulphur Below 0.05

the remainder being substantially iron.

References Cited UNITED STATES PATENTS 3,552,950 1/1971 Rundell 75l28 w x 3,563,729 2/1971 Kovach 75--128 w 1,991,438 2/1935 Wohrman 75-128 w x 3,306,736 2/1967 Rundell 75 12s G 3,627,516 12/1971 136116161. a1. 75-128 G L. DEWAYNE RUTLE'DGE, Primary Examiner us. 01. X.R.

75-128 G, 128 N, 128 W, 134 F 

